Abundance and distribution of megafaunal invertebrates in NE Pacific submarine canyons and their ecological associations with demersal fishes

Camelia Bianchi

Submarine canyons can influence the physical, chemical, and biological processes of coastal regions, thereby creating ecologically important areas of elevated production. Megafaunal invertebrates are diverse, long-lived organisms that play important ecological roles in marine ecosystems. Recent observations have assessed that a subgroup of megafaunal invertebrates called structure-forming invertebrates (e.g., large deep-sea corals and sponges) may have a functional role as living components of habitat for demersal fishes by providing relief and adding complexity to substrata in continental shelf and slope regions. We examined megafaunal invertebrate composition, substratum complexity, and associations between fishes and structureforming invertebrates within three submarine canyons in the Northeast Pacific. We made 31 submersible dives at water depths ranging from 90 to 1358 m and classified substrata consisting of high-relief rock walls and outcrops to low-relief mud. Mud dominated the substrata in all three canyons; even in areas of high- and moderate-relief, such as rock ridge and boulders, sediment covered many surfaces. We identified 130,125 individual megafaunal invertebrates from 68 taxa and 7 phyla. A community composition analysis revealed that spatial patterns of habitat and distribution of invertebrates are likely driven by variation in invertebrate densities and high aggregations of unique fauna in one canyon relative to the others. For example, Myxoderma sea stars were the most abundant organisms in Astoria Canyon, hermit crabs in Ascension Canyon, and spot prawns (Pandalus platyceros) in Carmel Canyon. The differences in topography, geology, and location among our study sites indicated that a broad range of unique pockets of microhabitat were formed in canyons and were able to support high aggregations of the organisms best adapted to changes in the physical environment and rates of disturbance. Structure-forming invertebrates were associated with few demersal fishes in close proximity with most of the associations occurring at ≥1 m away. However, fishes were seen resting inside and underneath these invertebrates (e.g., foliose sponges, vase sponges, barrel sponges, shelf sponges, and black corals) which were among the largest of all observed structure-forming invertebrates in our study, suggesting that their shapes created a suitable hiding place. The nature of the coexistence of fishes and invertebrates in similar habitats warrants further examination to determine if structure-forming invertebrates play a functional role as living components of habitat. Nonetheless, deep-sea corals, sponges, and other invertebrates are important organisms and deserve adequate protection. Management policies that take into account the whole ecosystem should be considered.

Abundance and distribution of megafaunal invertebrates in NE Pacific submarine canyons and their ecological associations with demersal fishes

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